Astronomy and Time Measurement

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Astronomy and Time Measurement


Observing the Heavens . The Egyptians studied the movements of heavenly bodies for religious reasons. Cycles of astronomical phenomena were used to measure time and thus allowed the Egyptians to perform rites on the correct day and at the precise hour. By observing the movement of a particular star, especially its disappearance for a period and return to a starting point, the Egyptians could calculate when a festival for a particular god should be celebrated. Egyptian astronomers could predict the time of sunrise and the appearance of certain stars associated with deities. These observations allowed a ritual to start at exactly the correct time. The Egyptian descriptions of these observations and calculations were expressed in mythological language, and noticeable changes in their methods and descriptions can be traced through their long history.

Prehistoric Astronomy . Evidence from Nabta in the Western Desert suggests that the Egyptians tried to mark the summer solstice in circa 6000 b.c.e. A twelve-foot-wide circle of stones, purposely laid in a pattern, was constructed along the summer solstice sunrise-sunset line. Another set of upright stones marked the north-south axis. These constructions suggest that Egyptians of this period were already developing a calendar based on the movement of the sun, with a fixed point at the summer solstice.

Predynastic Astronomy . Two myths with origins in the Predynastic Period (circa 3100-3000 b.c.e.) suggest that the Egyptians knew that the point of sunrise and sunset moved along a north-south line through the year. They probably recognized that the northern end of this line was the place where the sun rose and set on the summer solstice (modern 21 June) and that the southern end of this line was the point where the sun rose and set at the winter solstice (modern 21 December). The two myths that described sunrise, sunset, and the disappearance of the sun at night are known as the Myth of Nut and the Birth of Re and the Myth of Re’s Night Journey.

Nut and the Birth of Re . The Egyptians described the Milky Way as a female goddess named Nut (pronounced like “newt”). The sun was associated with the god Re. The whole of the Milky Way was visualized as a woman’s body with outstretched arms and legs. Near the constellation Gemini (the twins) in the western sky, the Egyptians saw Nut’s head. In the constellation Cygnus (the swan) in the eastern sky, the Egyptians saw cross shapes formed by the stars. The Egyptians re-created these shapes to mark the genital area on female figurines. The bright star in Cygnus, which the Egyptians called Deneb, was associated with the birth canal.

Movements of the Sun . The Egyptians also described the rising and setting of the sun in mythological terms. The god Re, the sun, was swallowed by Nut, whose head was in the western sky each evening at sunset. During the night, Re passed through Nut’s body. In the morning, at sunrise, Re was reborn through the star they called Deneb in the eastern sky. Though this swallowing and rebirth was a daily occurrence, the “original” swallowing of the sun took place at the vernal equinox (21 March) when the constellation Gemini, Nut’s head, appeared to drop below the western horizon and the “mouth” appeared to be located exactly where the sun had disappeared. The sun was born 272 days later (38.8 weeks, the average time from human conception to birth) through Deneb. The period from the swallowing, or conception, of Re to his birth is exactly the period from 21 March to 21 December. When Re was born, Deneb was thus located exactly at its southernmost point, the winter solstice. The myth of Nut and Re thus was used to describe certain astronomical phenomena and gave them a “cause.” The myth accurately described the movements of the stars, yet the mythological explanation relied on religious imagery to describe what was happening.

The Myth of Re’s Night Journey . The Egyptians needed to know the exact hour of sunrise so the rituals for Re could start exactly on time. They observed that twenty-four different clusters of stars, one bright and others dimmer, consistently rose on the horizon one hour before sunrise at different times of year. Each cluster’s rise accurately marked the moment one hour before sunrise for a fifteen-day period each year. Thus, the Egyptians worked out a sequence of twenty-four star clusters that could be observed to “foretell”

the sunrise. Each cluster had a known fifteen-day period—the half month—when it accurately foretold that the sun would rise one hour later. Only twelve of these twenty-four clusters were visible at any one time of the night in any one season of the year. The twenty-four clusters came to be associated with the hours of the day and night. These observations resulted in a day and night divided into twenty-four equal units, which we call hours.

Book of Gates . The Egyptians described these observations in the Book of Gates. This text gives an account of the Myth of Re’s Night Journey, in which Re passed through twelve doorways, one for each of the twelve hours of the night. Demons (dim stars) and a chief gatekeeper (the bright star) guarded each gate. Only knowledge of the correct magic spell would allow Re to pass through the gate. Later the myth and the Book of Gates were adapted for kings and others who, after death, approached the gods through the gates. When the Book of Gates was written down, these gates were regarded as a series of twelve nighttime hours. Since some versions of the Book of Gates have eight or ten hours, rather than twelve, some locations must have used longer “hours,” dividing the same amount of time into fewer units.

Earliest Calendar. A calendar needs at least one fixed, astronomical point that can be used to begin the count of 365 days, until that point is reached again. The Egyptians used two different astronomical phenomena to fix the beginning of the year. The fact that they used two points rather than one has suggested to scholars that originally there were two different calendars that were later combined. The two start points seem to be the Birth of Re (21 December, the winter solstice) and the reappearance of the star Sirius (19 July, its heliacal rising) one hour before the sunrise after a seventy-day disappearance from the sky. This latter event correlated loosely with the rising of the Nile flood.

Balancing the Year. The winter solstice, the day when the sunrise occurs at its southernmost point of the year, was called the Birth of Re. The Egyptians recognized early that 365 days passed between repetitions of this phenomenon. In Heliopolis, priests at the temple of Re used a calendar of 354 days—based on 12 lunar months. These months averaged 29 or 30 days. Each month the priest announced when he had observed the new moon. Such announcements were the only way to determine when the new month had started. Because each year lacked 11 days of the needed 365 separating the annual Birth of Re, every second or third year the priests added an extra month of 22 or 33 days. This procedure kept the Birth of Re in the same month every year, if not on the same day.

Timing the Flood. The second Predynastic calendar was related to the cult of the goddess Satet at her temple on Elephantine Island on the southern border of Egypt. The calendar was fixed on a celestial event the Egyptians called the Going Forth of Sopdet (the star, Sirius). On 19 July this star reappeared just before sunrise after a 70-day disappearance from the sky. This event loosely coincided with the beginning of the annual Nile flood. The Egyptians needed to predict the flood to time the planting and harvesting of their crops. The calendar was based on a 365-day year.

Early Dynastic Period. Sometime during Dynasties 1 and 2 (3000-2675 b.c.e.) in the Early Dynastic period the two calendars were combined, probably soon after the unification of Upper and Lower Egypt. The religious explanation for this combination was that the star Sopdet represented the goddess Isis, the daughter of Re. Perhaps because both Sopdet and Deneb rose near each other on their festivals, the priest could link the stars in one myth. Egyptologists call this Early Dynastic construction the religious calendar. It regulated festivals by reference to both the Birth of Re and the Going Forth of Sopdet. A major drawback of this calendar was the need to announce the new month based on observation. It was difficult for people outside the temple to know in advance whether a particular month would have twenty-nine or thirty days. The number of days assigned to each month depended on observing the moon.

The Civil Calendar. Scholars speculate that a more widespread need to know the actual date led to regularizing the calendar. Some time rather early in Egyptian history the 12 months were standardized to have 30 days. This organization established a year that contained 360 days. The additional 5 days needed to keep the Birth of Re on the winter solstice and the Going Forth of Sopdet near the start of the flood were added between the end of one year and the beginning of the next. Four months were included in each of three seasons. This new system is known as the civil calendar, from which anyone could calculate the proper date because all the months had the same number of days.

Astronomy and Old Kingdom Architecture . The major funeral monuments of the Old Kingdom reflect the relationship between religious ritual and astronomy. Both the pyramids at Giza, built in Dynasty 4 (circa 2625-2500 b.c.e.), and the Sun Temples, built by the kings of Dynasty 5 (circa 2500-2350 b.c.e.), were constructed to allow star observation, which controlled time measurement and insured that rituals were celebrated at the proper time. The Pyramids were both tombs and symbolic staircases allowing the king’s soul to climb to heaven and join with the stars. The deceased king then became one of the circumpolar stars that the Egyptians called the ikhemu-sek, “the ones that do not know destruction.” These stars never disappeared from the sky. All three Giza pyramids have north entrances that slope down so that the circumpolar stars are visible from the interior of the structure. The orientation of the pyramids to each other thus makes sense. They are not, therefore, arranged in a straight row, because this placement would block the view of the circumpolar stars.

Predicting Sunrise. The valley temples attached to the Sun Temples of Dynasty 5 also had a close connection to astronomy. These structures were aligned so that the axis

of each temple was oriented to the northeastern horizon, which allowed the roof to serve as an observatory for marking the nighttime hours. Userkaf’s temple (circa 2500-2485 b.c.e.) was oriented to a group of stars that included Deneb, the site of the birth of Re. All six of the known valley temples were used to predict the arrival of sunrise, although each used different groups of stars. Thus, they would have had different nighttime hours of varying lengths.

Middle Kingdom Timekeeping . Evidence from the Middle Kingdom (circa 1980-1630 b.c.e.) suggests that the Egyptians wanted to regularize the calculation of the hours. They searched for new methods of timekeeping that would use hours of equal lengths during the night and would start and end simultaneously everywhere. The evidence comes from star clocks that were inscribed on coffin lids dating to Dynasties 9 and 10 (circa 2130-1980 b.c.e.). These star clocks depicted thirty-six rising stars, called decans, that marked a night hour equivalent to forty minutes on a modern clock. The principal star was Sirius, already an important marker from the previous civil calendar. All the decans disappear from the sky for seventy days, then first return to view just before sunrise (helical rising). Each rising of a star pinpointed the start of a new ten-day week on the civil calendar. Three of these weeks formed one month. After each star reappeared, it joined the others that were now visible. At any one time there were eighteen visible stars, which were spaced in one-hour intervals across the sky. This system created a clock consisting of eighteen Middle Kingdom hours at night, which is equivalent to the modern twelve hours. Scholars have not yet identified most of the decans, with the exception of Sirius, although stars from the constellations Orion and the Big Dipper also were decans.

Synchronized Calendar . Though this system regularized the length of nighttime hours and made the hours simultaneous everywhere, a major problem remained. This system accounted for only 365 days in a year. The true astronomical year, of course, is 365.25 days. Thus, after four years the calendar of stars was one day off the true astronomical year. After 120 years, the calendar deviated a full month from the actual seasons. The Egyptians recognized this problem and attempted to reset the star clocks regularly throughout the Middle Kingdom to keep the seasons and the calendar synchronized.

New Kingdom Astronomy . A new star clock consisting of twelve night hours was recorded in royal tombs during Dynasty 20 (circa 1190-1075 b.c.e.). This clock was probably calibrated with a water clock, known to be in use by the beginning of Dynasty 18 (circa 1539 b.c.e.), about 380 years earlier. The new system of timekeeping was based on a series of twelve constellations crossing an imaginary line drawn in the sky. To make the system work, two priests observed the night sky while sitting facing each other. One priest could watch the star group “pass” the other’s shoulder or ear. He then could announce that the next hour had started. This system was more accurate than previous night-hour calculations because it was calibrated to a water clock, which resembled a large vase with a small hole in the bottom. Priests filled it each night and the water gradually escaped through the hole. Water-level measurements, carved into the side of the clock, marked the hours of the night. Daytime shadow clocks, also in use in the New Kingdom, were accurate from about 8 A.M. to 4 P.M.—any earlier or later in the day and the shadows were too long to record accurately. No major advances in astronomy are known until after Alexander the Great invaded Egypt, when local astronomy was combined with Greek and Babylonian sciences to make further advances.


R. A. Wells, “The 5th Dynasty Sun Temples at Abu Ghurab as Old Kingdom Star Clocks; Examples of Applied Ancient Egyptian Astronomy,” Beihefte zu Studien zur Altägyptischen Kultur, Akten des Vierten Internationalen Ägyptologen Kongresses München 1985, volume 4 (1990): 95-104.

Wells, “The Mythology of Nut and the Birth of Ra,” Studien zur Altägyptischen Kultur, 19 (Hamburg: H. Buske Verlag, 1992): 305-321.

Wells, “Origin of the Hour and the Gates of the Duat,” Studien zur Altägyptischen Kultur, 20 (Hamburg: H. Buske Verlag, 1993): 305-326.

Wells, “Re and the Calendars,” in Revolutions in Time: Studies in Egyptian Calendrics, edited by Anthony John Spalinger (San Antonio: Van Siclen Books, 1994), pp. 1–28.

Wells, “Sothis and the Satet Temple at Elephantine: A Direct Connection,” Studien zur Altägyptischen Kultur, 12 (Hamburg: H. Buske Verlag, 1985): 255-302.

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